Lars,
Normal Cats and especially Power Cats have an awful behaviour in rough seas. There is an alternative called SWATH (Small Waterplane Area Twin Hull), an innovative hull concept for smooth service in rough seas. The cross section at sea level is minimized and thus only a minimum is exposed to the lifting forces of the waves. Go to Abeking & Rasmussen to explore more about the SWATH. There you can even see movies comparing the behaviour of monohulls and a SWATH
http://www.abeking.com
Regards
Bernd